1. Technical Field of the Invention
The present invention relates generally to automotive alternators for use in motor vehicles, such as passenger cars and trucks. More particularly, the invention relates to an automotive alternator which includes an improved magnet holder for holding a plurality of permanent magnets between interleaved claw poles of a pair of Lundell-type pole cores.
2. Description of the Related Art
There is known a method of improving the efficiency of an automotive alternator, which includes a rotor having a pair of Lundell-type pole cores, by disposing a plurality of permanent magnets between the pole cores.
More specifically, each of the pole cores is secured on a rotating shaft of the rotor and includes a plurality of claw poles extending in the axial direction of the rotating shaft. The claw poles of one of the pole cores are alternately arranged with those of the other pole core in the circumferential direction of the rotating shaft. Moreover, each circumferentially-adjacent pair of the claw poles of the pole cores are so magnetized as to have opposite polarities. As a result, leakage of magnetic flux may occur between each circumferentially-adjacent pair of the claw poles. In view of the above, according to the method, each of the permanent magnets is interposed between a circumferentially-adjacent pair of the claw poles of the pole cores, so as to reduce the leakage of magnetic flux between the pair of the claw poles and to direct the magnetic flux developed by the permanent magnet toward a field coil of the alternator, thereby improving the efficiency of the alternator.
Furthermore, for each of the permanent magnets, the centrifugal force acting on the permanent magnet is generally sustained by ledge portions formed in the claw poles adjacent to the permanent magnet. However, in addition to the centrifugal force, the permanent magnet also receives an urging force, which urges the permanent magnet in the circumferential direction during acceleration of the alternator, and an exciting force caused by a harmonic magnetic field or external vibration. Therefore, it is necessary to fix the permanent magnet between the adjacent claw poles, thereby restricting movement of the permanent magnet in both the radial and circumferential directions of the rotating shaft. In addition, the permanent magnet is generally brittle, and it is thus preferable to use a magnet holder made of a non-magnetic material to enclose the permanent magnet, thereby protecting the permanent magnet; it is further preferable to bond the permanent magnet to the inner surface of the magnet holder.
Japanese Patent First Publication No. 2008-54392, an English Equivalent of which is US Patent Application Publication NO. 2008/0048516 A1, discloses a permanent magnet protection mechanism which includes a plurality of magnet holders and a plurality of connecting members. Each of the magnet holders holds one of a plurality of permanent magnets. Each of the connecting members connects an adjacent pair of the magnet holders. All of the magnet holders and connecting members are integrally formed by punching and bending a band plate of nonmagnetic stainless steel. With the integral formation, it is possible to eliminate the step of connecting or bonding the connecting members to the magnet holders, and to achieve a reliable connection between the connecting members and the magnet holders.
However, in the above permanent magnet protection mechanism, the magnet holders are shaped to fit to the side faces of claw poles of a pair of Lundell-type pole cores. Consequently, the circumferential interval between each adjacent pair of the magnet holders becomes very small at the tip of the claw pole interposed therebetween. Accordingly, in the band plate, the longitudinal interval between each adjacent pair of those portions which subsequently make up the magnet holders is very small at one of the lateral ends of the band plate. Consequently, with the small longitudinal interval, it is difficult to form, in the subsequent punching and bending steps, sufficiently large side faces of the magnet holders. As a result, without sufficiently large side faces, it is difficult for the magnet holders to completely cover the side faces of the permanent magnets; thus, it is difficult for the magnet holders to securely hold and protect the permanent magnets.
Moreover, to completely cover all of the radial end faces and axial end faces of the permanent magnets, it is necessary for the band plate to have a width which is greater than twice the sum of the length and height of the permanent magnets. However, with such a large width of the band plate, the yield rate at the lateral ends of the band plate becomes very low.
Furthermore, in the above permanent magnet protection mechanism, each of the connecting members connects an adjacent pair of the magnet holders at the base of the claw pole interposed between the pair of the magnet holders. Consequently, the connecting members are alternately arranged at the lateral ends of the permanent magnet protection mechanism in the longitudinal direction of the permanent magnet protection mechanism (i.e., the circumferential direction of the rotating shat).
In mass production, the permanent magnet protection mechanism is generally not immediately mounted to the pole cores, but temporality stored in a storage box along with other permanent magnet protection mechanisms. However, with the arrangement of the connecting members as described above, the permanent magnet protection mechanism may be easily entwined with the other permanent magnet protection mechanisms in the storage box. Consequently, it may be difficult to take out, without deformation, the permanent magnet protection mechanism from the storage box.